Heterologous interactions between adenylylated and undenylylated subunits of glutamine synthetase do not affect the catalytic potential (Vmax) of unadenylylated subunits, but they do affect the affinity of adenylylated subunits for the substrate glutamate. The capacity of several monovalent cations to activate or inhibit the biosynthetic activity of glutamine synthetase indicates that there is at least one relatively non specific binding site on the enzyme for monovalent cations that leads to activation of the enzyme and in addition there is an inhibitor site that is relatively specific for Cs ions, Li ions and Rb ions. Each subunit of glutamine synthetase possesses two kinds of binding sites for amino acids. One site is non-specific for all D-amino acids and the other is specific for L-alanine and L-serine; glycine can bind to both sites.